A third of all Windows users -- including up to 20 percent of enterprise systems -- could still be running XP when Microsoft pulls the plug on patches in 2014

The decline in usage share of Windows XP, which is slated for retirement in 53 weeks, has slowed significantly, hinting that millions of its users will hold onto the operating system much longer than some, including Microsoft, expect.

In the past three months, Windows XP's monthly drop in share has averaged just 0.12 of a percentage point. That's less than a fifth as much as the 12-month average of 0.68 percentage points.

Other averages point to a major deceleration in declining usage share: XP's most recent six-month average decrease of 0.42 percentage points was less than half the 0.94 point average for the prior six months.

Likewise for longer timespans. In the last 12 months, Windows XP has dropped an average of 0.68 percentage points, while in the 12 months prior it fell by 0.83 percentage points.

In other words, in the second half of a 12-month stretch, XP's decline slowed by 55 percent; in the second year of a two-year span, it slowed 18 percent.

The slowdown paints a picture that must depress Microsoft, which has been banging the upgrade drum at Windows XP users for nearly two years, and has repeatedly warned them that free security updates will stop after April 8, 2014.

Net Applications' data can also be used to roughly plot XP's future usage share.

If the average decline of the last 12 months holds, XP will still account for 30 percent of all personal computers at the end of April 2014, or 33 percent of all systems expected to be running Windows at that time.

Recent estimates of XP's future by analysts, however, have been more conservative, with experts from Gartner and Forrester Research predicting that 10 percent to 20 percent of enterprise systems will still be on the aged OS when support stops.

Microsoft has not pegged XP's current corporate share, but the Redmond, Wash., software developer clearly knows it's large: In January, during the company's last quarterly earnings call, CFO Peter Klein said 60 percent of all enterprise PCs were running Windows 7.

Since few businesses adopted Windows Vista -- and with Vista's usage share now under 5 percent, some that did likely ditched it -- the remaining 40 percent must, by default, largely be Windows XP.

Windows XP will not suddenly stop working 53 weeks from now; it will boot, run applications and connect to the Internet as it did before. But it will not be served with security updates. Minus patches, and knowing how frequently cyber criminals uncover vulnerabilities, security experts expect hackers to exploit XP bugs that users will have no way of quashing.

Those same experts have split on whether Microsoft will extend Windows XP's support to protect what increasingly looks to be a major chunk of Windows users. But Microsoft has not signaled any desire to do so.

Granted, Microsoft will have supported XP for 12 years and 5 months, or about two-and-a-half years longer than its usual decade. That will be a record, as XP this month tied the previous Methuselah, Windows NT, which received 11 years and five months of support.

But Microsoft could still rethink its XP policy, and mimic rival Apple, which has continued to support OS X Snow Leopard, an operating system that, like XP, maintains a robust usage share.

Apple, which has never spelled out its security update policies, typically has stopped supporting "n-2," where "n" is the most current edition of OS X, around the time it releases "n."

Snow Leopard -- "n-2" in that formula, having been superseded by Lion and Mountain Lion, the latter representing "n" -- has continued to receive security updates, most recently on March 14, or about eight months after Mountain Lion's launch.

By continuing to update Snow Leopard, which powered 27 percent of all Macs last month, Apple patched 91 percent of all Macs last month.

Microsoft could do even better -- cover 96 percent of all current Windows PCs -- by continuing to support XP after April 2014.

But one expert thought that very unlikely. "I think they have to draw a line in the sand," said John Pescatore, then an analyst with Gartner, now with the SANS Institute, in an interview last December. "They've supported XP longer than anything else, so they'd be pretty clean from the moral end."

To track how long XP has before retirement, users can browse to an online countdown clock maintained by Camwood, a U.K. firm that specializes in helping businesses migrate to newer operating systems.